1. Field of the Invention
This invention relates to a magneto-optical recording system of the magnetic field modulation mode and a modulated magnetic field generating apparatus used therein.
2. Optical Art
Optical disks are of great interest as high capacity information carrying media. Among the optical disks, magneto-optical disks of the magnetic field modulation mode are expected to find use in data files and other advanced applications. The magnetic field modulation mode is to record signals in a magneto-optical disk by directing a laser beam from an optical head to the recording layer in a DC manner for raising the temperature of the irradiated spot, and applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head at the same time. Therefore, the magnetic field modulation mode allows for overwrite recording and has potential application to rewritable compact disks.
The magnetic heads used in the magnetic field modulation mode are classified into flying magnetic heads which are kept afloat on the disk surface by aerodynamic buoyancy due to disk rotation like the magnetic heads of rigid disk drives and non-flying magnetic heads which are positioned at a fixed distance from the disk surface independent of disk rotation. The non-flying magnetic heads include a stationary head which is fixedly secured at a fixed distance from the disk surface and a sliding head which is received in a slider such that the head is backwardly remote from the disk sliding surface of the slider whereby the head is kept at a fixed distance from the disk surface. The non-flying magnetic head can apply a stronger magnetic field with ease since the head is closer to the disk. The stationary magnetic head has the risk of collision with the disk because the distance between the head and the disk can vary due to axial runouts of the rotating disk due to its warpage and strains. Sometimes a displacement sensor and actuator are provided for keeping the head-disk spacing constant. In contrast, the sliding head is easy to maintain the head-disk distance constant.
Since the non-flying head performs independent of the revolution of the disk, it is adequate for those disks which are operated at a rotational speed as low as compact disks (CD). The non-flying head is relatively free of troubles associated with projections and deposits on the disk surface. If the displacement sensor and actuator can be omitted, it is of simple structure and can be manufactured at a low cost.
However, the non-flying magnetic head for use in a magneto-optical recording system has to apply a magnetic field of higher intensity at a spot remote from the head because the spacing between the head core and the recording medium is increased as compared with flying magnetic heads of rigid disk drives. It is then necessary to conduct electric current as large as about 1 Ap-p across the coil on the head core, with the increased current flowing across the overall modulated magnetic field generating circuit. It is further necessary to reverse such increased current flow at a high frequency. The frequency of current reversal is about 720 kHz at maximum when EFM signals are recorded at a linear speed of 1.4 m/s as used with conventional compact disks (CD) and mini disks (MD). A drive is under development which carries out write/read operation at a linear speed twice higher than the conventional speed for increasing a data transfer rate. With such an advanced drive, the frequency of current reversal can be further increased.
For increasing the current switching speed, JP-A 3-157839 proposes a circuit including two coils and switching elements combined therewith as shown in FIG. 1 wherein the frequency of current reversal is increased by utilizing the constant current effect of inductive reactance.
Japanese U.M. Application Kokai No. 5-15102 intends to improve the circuit of JP-A 3-157839 by adding a damping resistance thereto for the following reason. Since a switching element has an electrostatic capacity, interaction can occur between the capacitance of a switching element and the inductance of a magnetic head coil upon switching of the switching element from on to off. Then a transient current flow having an excessive overshoot is introduced in the driving current flow to adversely affect a reversal time, which is reflected by the recording magnetic field to degrade recording properties. The provision of a damping resistance, however, increases the power consumption of the circuit.